1. Field of the Invention
The present invention relates to a fast transient response PWM control apparatus for a voltage regulator with adaptive voltage position and its driving signal generating method. In particular, this invention is used for a switching DC—DC power converter apparatus. The present invention detects changes in the loading instantaneously and creates a response rapidly to stabilize the output voltage.
2. Description of the Related Art
Control methods for a switching DC—DC power converter are typically divided into two types. One uses clock signals as the frequency for controlling the switching of the power devices. Another one does not use a clock signal to control the switching of the power devices in the DC—DC converter. When using a clock signal, the frequency of the switching signal of the DC—DC converter is fixed and is the same as the frequency of the clock signal. The duty cycle (D) of the switching signal, defined as
      D    =                  T        on                              T          on                +                  T          off                      ,varies according to the feedback control, moving either the leading-edge or the trailing edge of the signal to achieve pulse width modulation (PWM), For the modulation using a fixed clock signal to generate the switching signal, the modulation of the switching signal is limited due to the response time of the clock signal because even the fastest response change still need to wait for one cycle. Hence the switching signal cannot respond to the change of the loading instantaneously if the loading changes rapidly. Furthermore, the feedback loop, which controls the duty cycle change speed, is related to the control bandwidth. The latter has to be designed based on trade-off with the stability requirement.
For solving the above problem, another technology for controlling the switch of the DC—DC converter does not need the clock signal. It adopts a hysteretic control method or a constant on-time control method.
Please refer to FIG. 1, which shows a schematic block diagram of the DC—DC power converter having a hysteretic control of the prior art. The DC—DC power converter having a hysteretic control 1 is composed of a hysteretic comparator 10 connected with a driving unit 12, an energy storage inductor L and an energy storage capacitor C. The hysteretic comparator 10 obtains a feedback signal Vfb from the output port of the DC—DC power converter circuit 1 and compares it with a hysteretic boundary value VH to output a driving signal Vdriver. The driving signal Vdriver is used for controlling a switch (not shown in the figure) of the driving unit 12 and a stable output voltage Vout is generated according to an input voltage Vin.
Please refer to FIGS. 1 and 2. FIG. 2 shows the typical waveforms of the DC—DC power converter having a hysteretic control of the prior art. When the feedback signal Vfb on decreasing reaches the lower boundary value −VH of the hysteretic boundary value VH, the driving signal Vdriver is changed from a low level to a high level. When the feedback signal Vfb on increasing reaches the upper boundary value +VH of the hysteretic boundary value VH, the driving signal Vdriver is changed from a high level to a low level. Via the above method, a driving signal is generated. The feedback signal for the hysteretic control is typically the output voltage, which has ripples due to the effective series resistor (ESR) and pulsating charging and discharging currents. The generated ripples affect the accurate operation of the hysteretic comparison and make the switching frequency of the driving signal Vdriver change rapidly. When the level of the ripples is small, the disturbance of the noise is serious and the method does not work well in low ripple and high-accuracy application.
Please refer to FIG. 3, which shows a schematic block diagram of the DC—DC power converter having a constant on-time control of the prior art. The DC—DC power converter circuit usually adopts a voltage control mode. Using a buck DC—DC power converter 2 as an example, in the DC—DC power converter 2, the control circuit 20 uses an error amplifier 23 to obtain a feedback voltage signal VFB. Then, the feedback voltage signal VFB is compared with a reference voltage Vref to amplify and output an error signal VE. A PWM comparator compares the error signal VE and an integration output signal Vramp to output a PWM setting signal PWMset to a flip-flop 26 for enabling an output driving signal PWMDRV of the flip-flop 26.
The enabled output-driving signal PWMDRV controls an on-time control unit 28 to delay a fixed period and output a delay signal DelayOn. The delay signal Delay-On controls the reset of the flip-flop 26 via an OR gate 27 and obtains an output-driving signal PWMDRV having a fixed on-time at the output port Q of the flip-flop 26. The output driving signal PWMDRV outputs a pair of driving signals DrvH and DrvL that are complementary via a driving unit 29. The complementary driving signals DrvH and DrvL is used for driving the switch of transistors Q1 and Q2. By a pulse width modulation method, the output voltage Vout of a power output circuit 22 is stable.
The control circuit 20 includes an over-current protection unit 24 for receiving a current detection signal Vs. The current detection signal Vs is compared with a critical signal Vthocp to output an over-current signal OCPen. The over-current signal OCPen and the delay signal DelayOn pass through the OR gate 27 to control the reset of the flip-flop 26 for obtaining the output-driving signal PWMDRV having a fixed on-time.
Please refer to FIG. 4, which shows waveforms diagram of the circuit block in the FIG. 3. The error signals VE is compared with the integration output signal Vramp to output a PWM setting signal PWMset for enabling the driving signal PWMDRV. The PWM setting signal PWMset enables the on-time control unit 28. After a fixed delay time Ton, the on-time control unit 28 resets the driving signal PWMDRV.
Please refer to FIG. 3 again. In the fixed on-time control method, the on-time for switching period driving signal is fixed and the off-time is controlled by the modulation of the error amplifier 23. In other words, the fixed on-time makes the modulation of the off-time become influenced by the change of the output or input voltage of the DC—DC power converter 2. Therefore, the switching frequency is changed. In the buck DC—DC power converter 2, the switching frequency fs is affected by the output voltage Vout and the input voltage Vin when the on-time is fixed. Please refer to formula (1).
                    fs        =                  Vout                      Ton            ×            Vin                                              (        1        )            
Furthermore, the fixed on-time control method needs to use the error amplifier 23 to control the modulation. The transient response of the control signal is affected by the limitations of the bandwidth of the error amplifier 23.